A speed ratio control mechanism or lockup mechanism of an automatic transmission are controlled using an oil pressure (line pressure) supplied from an oil pump to the automatic transmission of a vehicle. The oil pump is driven in synchrony with the rotations of the engine. In such a device, the rotation speed of the pump is controlled with respect to a determined necessary pump discharge amount considering the characteristics of the speed ratio control mechanism and the response time to establish an oil pressure.
However the engine rotation speed undergoes large variations due to driving conditions and as a result the pump rotation speed also varies. Even at low rotation speeds such as when the engine is idling, it is necessary for the pump to be relatively large in order to maintain the necessary pump discharge amount. In particular, a continuously variable transmission requires a large pump discharge amount to lubricate the variable pulleys, the drive force transmission mechanism or the like, in addition to the required flow amount for the speed ratio control mechanism. As a result, it has been necessary to provide a pump with a large capacity to satisfy such required pump discharge amount at low engine rotation speeds.
On the other hand, at high rotation speeds, the pump discharge amount will often become excessive and unnecessary discharges will result. This has resulted in the problem that pump driving energy is wasted. To that degree, this has an adverse effect on engine fuel consumption and has also resulted in increase in pump noise.